The present invention relates to a reference voltage generation circuit and reference current generation circuit in a semiconductor device, and more particularly to a reference voltage generation circuit and reference current generation circuit constituted by MOS transistors in a semiconductor device using, for example, a reference voltage lower than the power supply voltage.
A band gap reference (BGR) circuit has been known as a less temperature-dependent, less power-supply-voltage-dependent reference voltage generation circuit. The name of the circuit has come from generating a reference voltage almost equal to the silicon's bandgap value of 1.205V. The circuit is often used to obtain highly-accurate reference voltages.
With a BGR circuit constituted by conventional bipolar transistors in a semiconductor device, the forward voltage (with a negative temperature coefficient) at a p-n junction diode or the p-n junction (hereinafter, referred to as the diode) between the base and emitter of a transistor whose collector and base are connected to each other is added to a voltage several times as high as the voltage difference (having a positive temperature coefficient) of the forward voltages of the diodes differing in current density in order to output a voltage of about 1.25V with a temperature coefficient of nearly zero.
At present, the voltage on which semiconductor devices operate is getting lower. When the output voltage of a BGR circuit was about 1.25V, the lower limit of the power supply voltage was 1.25V+.alpha.. Consequently, however small .alpha. may be made, the semiconductor device could not be operated on the power supply voltage of 1.25V or lower.
The reason for this will be explained in detail.
FIG. 1 shows the basic configuration of a first conventional BGR circuit constituted by n-p-n transistors.
In FIG. 1, Q.sub.1, Q.sub.2, and Q.sub.3 indicate n-p-n transistors, R.sub.1, R.sub.2, and R.sub.3 resistance elements, and I a current source. Furthermore, V.sub.BE1, V.sub.BE2, and V.sub.BE3 represent the base-emitter voltages of the transistors Q.sub.1, Q.sub.2, and Q.sub.3 respectively, and V.sub.ref the output voltage (reference voltage).
When the transistors Q.sub.1, Q.sub.2 have the same characteristics, the emitter voltage V.sub.2 of the transistor Q.sub.2 is: EQU V.sub.2 =V.sub.BE1 -V.sub.BE2 =V.sub.T.multidot.ln(I.sub.1 /I.sub.2) (1)
This gives:
V.sub.ref =V.sub.BE3 +(R.sub.3 /R.sub.2)V.sub.2 EQU =V.sub.BE3 +(R.sub.3 /R.sub.2)V.sub.T.multidot.ln(I.sub.1 /I.sub.2) (2)
The first term in equation (2) has a temperature coefficient of about -2 mV/.degree. C. In the second term in equation (2), the thermal voltage V.sub.T is: EQU V.sub.T =k.multidot.T/q (3)
Thus, the temperature coefficient is expressed as: EQU (R.sub.3 /R.sub.2)(k/q)ln(I.sub.1 /I.sub.2) (4)
To find the condition for making the temperature coefficient of V.sub.ref zero, substituting EQU k=1.38.times.10.sup.-23 J/K (5) EQU q=1.6.times.10.sup.-19 C (6)
This gives: EQU (R.sub.3 /R.sub.2)ln(I.sub.1 /I.sub.2)=23.2 (7)
In equation (2), if V.sub.BE3 =0.65V at 23.degree. C., EQU then V.sub.ref =0.65+0.6=1.25V (8)
This value is almost equal to the bandgap value (1.205) of silicon.
The BGR circuit of FIG. 1 has disadvantages in that its output voltage is fixed at 1.25V and its power supply voltage cannot be made lower than 1.25V.
FIG. 2 shows the basic configuration of a second conventional BGR circuit using no bipolar transistor.
The BGR circuit is constituted by a diode D.sub.1, an N number of diodes D.sub.2, resistance elements R.sub.1, R.sub.2, R.sub.3, a differential amplifier circuit DA.sub.1 constituted by CMOS transistors, and a PMOS transistor T.sub.p.
The voltage V.sub.A at one end of the diode D.sub.1 is supplied to the--side input of the differential amplifier circuit DA.sub.1 and the voltage V.sub.B at one end of the diode D.sub.2 is supplied to the +side input of the circuit DA.sub.1, so that feedback control is performed such that V.sub.A is equal to V.sub.B (the voltages at both ends of R.sub.1 is equal to those of R.sub.2). EQU Thus, I.sub.1 /I.sub.2 =R.sub.2 /R.sub.1 (9)
The characteristics of the diode are expressed by the following equations: EQU I=Is{e.sup.(q.multidot.V F.sup./k.multidot.T) -1} (10) EQU V.sub.F &gt;&gt;q/k.multidot.T=26 mV (11)
where Is is the (reverse) saturation current and V.sub.F is the forward voltage.
From equation (11), -1 in equation (10) can be ignored. This gives: EQU V.sub.F =V.sub.T.multidot.ln(I/Is) (12)
The voltage across the resistance element R.sub.3 is: EQU dV.sub.F =V.sub.F1 -V.sub.F2 =V.sub.T.multidot.ln(N.multidot.I.sub.1 /I.sub.2) EQU =V.sub.T.multidot.ln(N.multidot.R.sub.2 /R.sub.1) (13)
The thermal voltage V.sub.T has a positive temperature coefficient k/q=0.086 mV/.degree. C. and the forward voltage V.sub.F1 of the diode D.sub.1 has a negative temperature coefficient of about -2 mV/.degree. C.
Then, under the following conditions: EQU V.sub.ref =V.sub.F1 +(R.sub.2 /R.sub.3)dV.sub.F (14) EQU {character pullout}V.sub.ref /{character pullout}T=0 (15)
the resistance values of the resistance elements R.sub.1, R.sub.2, and R.sub.3 are set.
As an example, if N=10, R.sub.1 =R.sub.2 =600 k.OMEGA., and R.sub.3 =60 k.OMEGA., dV.sub.F will be the voltage difference between diode D.sub.1 and diode D.sub.2 whose current ratio is 1:10. This will give: EQU V.sub.ref =V.sub.F1 +10.multidot.dV.sub.F =1.25V (16)
Like the first conventional circuit, the second conventional circuit has disadvantages in that its output voltage is fixed at 1.25V (or invariable) and the power supply voltage used cannot be made lower than 1.25V.
As described above, conventional BGR circuits that generate a less temperature-dependent, less power-supply-voltage-dependent reference voltage have disadvantages in that their output voltage is fixed at about 1.25V and they cannot be operated on a power supply voltage lower than about 1.25V.